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package org.apache.lucene.util;
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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import java.lang.reflect.Method;
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import java.util.Collections;
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import java.util.HashSet;
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import java.util.WeakHashMap;
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* A utility for keeping backwards compatibility on previously abstract methods
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* (or similar replacements).
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* <p>Before the replacement method can be made abstract, the old method must kept deprecated.
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* If somebody still overrides the deprecated method in a non-final class,
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* you must keep track, of this and maybe delegate to the old method in the subclass.
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* The cost of reflection is minimized by the following usage of this class:</p>
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* <p>Define <strong>static final</strong> fields in the base class ({@code BaseClass}),
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* where the old and new method are declared:</p>
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* static final VirtualMethod<BaseClass> newMethod =
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* new VirtualMethod<BaseClass>(BaseClass.class, "newName", parameters...);
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* static final VirtualMethod<BaseClass> oldMethod =
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* new VirtualMethod<BaseClass>(BaseClass.class, "oldName", parameters...);
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* <p>This enforces the singleton status of these objects, as the maintenance of the cache would be too costly else.
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* If you try to create a second instance of for the same method/{@code baseClass} combination, an exception is thrown.
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* <p>To detect if e.g. the old method was overridden by a more far subclass on the inheritance path to the current
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* instance's class, use a <strong>non-static</strong> field:</p>
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* final boolean isDeprecatedMethodOverridden =
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* oldMethod.getImplementationDistance(this.getClass()) > newMethod.getImplementationDistance(this.getClass());
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* <em>// alternatively (more readable):</em>
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* final boolean isDeprecatedMethodOverridden =
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* VirtualMethod.compareImplementationDistance(this.getClass(), oldMethod, newMethod) > 0
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* <p>{@link #getImplementationDistance} returns the distance of the subclass that overrides this method.
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* The one with the larger distance should be used preferable.
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* This way also more complicated method rename scenarios can be handled
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* (think of 2.9 {@code TokenStream} deprecations).</p>
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public final class VirtualMethod<C> {
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private static final Set<Method> singletonSet = Collections.synchronizedSet(new HashSet<Method>());
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private final Class<C> baseClass;
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private final String method;
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private final Class<?>[] parameters;
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private final WeakHashMap<Class<? extends C>, Integer> cache =
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new WeakHashMap<Class<? extends C>, Integer>();
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* Creates a new instance for the given {@code baseClass} and method declaration.
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* @throws UnsupportedOperationException if you create a second instance of the same
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* {@code baseClass} and method declaration combination. This enforces the singleton status.
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* @throws IllegalArgumentException if {@code baseClass} does not declare the given method.
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public VirtualMethod(Class<C> baseClass, String method, Class<?>... parameters) {
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this.baseClass = baseClass;
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this.parameters = parameters;
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if (!singletonSet.add(baseClass.getDeclaredMethod(method, parameters)))
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throw new UnsupportedOperationException(
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"VirtualMethod instances must be singletons and therefore " +
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"assigned to static final members in the same class, they use as baseClass ctor param."
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} catch (NoSuchMethodException nsme) {
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throw new IllegalArgumentException(baseClass.getName() + " has no such method: "+nsme.getMessage());
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* Returns the distance from the {@code baseClass} in which this method is overridden/implemented
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* in the inheritance path between {@code baseClass} and the given subclass {@code subclazz}.
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* @return 0 iff not overridden, else the distance to the base class
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public synchronized int getImplementationDistance(final Class<? extends C> subclazz) {
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Integer distance = cache.get(subclazz);
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if (distance == null) {
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cache.put(subclazz, distance = Integer.valueOf(reflectImplementationDistance(subclazz)));
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return distance.intValue();
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* Returns, if this method is overridden/implemented in the inheritance path between
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* {@code baseClass} and the given subclass {@code subclazz}.
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* <p>You can use this method to detect if a method that should normally be final was overridden
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* by the given instance's class.
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* @return {@code false} iff not overridden
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public boolean isOverriddenAsOf(final Class<? extends C> subclazz) {
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return getImplementationDistance(subclazz) > 0;
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private int reflectImplementationDistance(final Class<? extends C> subclazz) {
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if (!baseClass.isAssignableFrom(subclazz))
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throw new IllegalArgumentException(subclazz.getName() + " is not a subclass of " + baseClass.getName());
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boolean overridden = false;
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for (Class<?> clazz = subclazz; clazz != baseClass && clazz != null; clazz = clazz.getSuperclass()) {
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// lookup method, if success mark as overridden
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clazz.getDeclaredMethod(method, parameters);
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} catch (NoSuchMethodException nsme) {
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// increment distance if overridden
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if (overridden) distance++;
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* Utility method that compares the implementation/override distance of two methods.
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* <li>> 1, iff {@code m1} is overridden/implemented in a subclass of the class overriding/declaring {@code m2}
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* <li>< 1, iff {@code m2} is overridden in a subclass of the class overriding/declaring {@code m1}
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* <li>0, iff both methods are overridden in the same class (or are not overridden at all)
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public static <C> int compareImplementationDistance(final Class<? extends C> clazz,
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final VirtualMethod<C> m1, final VirtualMethod<C> m2)
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return Integer.valueOf(m1.getImplementationDistance(clazz)).compareTo(m2.getImplementationDistance(clazz));